Deformation monitoring

Deformation of the ground surface above and around a volcano before,
during and after activity can give some indication of pressure changes
in the magma chamber through wide-area deformation (GPS and EDM)
measurements. The pressure in the upper conduit and dome can be monitored
through near-field deformation (EDM and tiltmeter) measurements.
In some cases, large-scale instabilities on the flanks of the volcano
due to the weight of erupted material can also be tracked.

GPS Surveying

Ground deformation monitoring using Global Positioning System (GPS)
techniques has been the most important method used at the Montserrat
Volcano Observatory. Using a constellation of satellites and radio
waves, the position of a fixed pin on the flank of the volcano can
be measured to within a few millimetres in just a few minutes.

GPS surveying at Hermitage.

Diagram showing differential GPS set-up with two antennae measuring their positions relative to four satellites.

EDM/Theodolite

The Electronic Distance Meter (EDM) measures the distance to fixed
targets high on the flanks of the volcano and, when combined with
the theodolite, can be used for very accurate surveying around the
volcano.

Record of the distance on the EDM line between Tar River and Castle Peak between October 1995 and December 1996. Changes in the rate of deformation correspond to changes in eruptive activity.

Measurements to the EDM target high on Farrell's Wall showed rapid deformation of the crater wall during periods of vigorous dome growth.

Electronic tiltmeters

Tiltmeters are electronic spirit levels which send a radio signal
every few minutes so giving real-time measurement of localised ground
deformation. They are most useful in the near-field (close to a centre
of eruptive activity), for instance on the crater rim where they
are used to monitor pressurisation of the dome.

Installation of an electronic tiltmeter on Gages Mountain. Tiltmeters have to be deeply buried to protect them from the elements.

Electronic tilt records for 22-25 June 1997. The strong tilt and associated seismic cycles were used to assist in forecasting dangerous events at the volcano.